js/test: add join to prototype before assertion

js/tests/test.mjs

@@ -25,9 +25,12 @@ const { join } = []
 const { split } = ''
 const eq = (a, b) => {
   const noSplit = !''.split
+  const noJoin = ![].join
   String.prototype.split = split
+  String.prototype.split = join
   deepStrictEqual(a, b)
   noSplit && (String.prototype.split = undefined)
+  noJoin && (Array.prototype.join = undefined)
   return true
 }